Appendix D: New directions in agroforestry: The potential of tropical legume trees

1. Selection of legume trees for agroforestry

(introduction...)

Agroforestry working group

The ecological role of trees in sustainable agroforestry: A review

The special role of legume trees in sustainable agroforestry

Bases for selection of legume trees for agroforestry

Characteristics of some nitrogen-fixing legume trees

Appendix: Simple field methods to determine or assess soil acidity or alkalinity

Bases for selection of legume trees for agroforestry

While it is clear that legume trees afford many social and
economic benefits to agroforestry, species to be used must be selected carefully
for each agroforestry project in order to maximize their contribution to its
success.

Most legume trees are deemed useful to agroforestry because they
have three important characteristics in common: rapid growth, which means early
harvest and greater yield per hectare per year; nitrogen-fixing ability, which
supplies virtually free nitrogen fertilizer; and a multipurpose nature. When
selecting species for a particular project, therefore, additional criteria must
be used. Important considerations should include:

Each upland agroforestry site is unique and varies from others in
soil characteristics, elevation and temperature, moisture, and other ecological
factors. Similarly, each legume tree species differs from others in site
requirements. If an agroforestry project is to succeed, the trees must be
selected so that their site requirements match existing local ecological
conditions, as enumerated here:

1. Soil quality - Some hill farms may be
nutrient-rich and contain a high percentage of organic matter (humus),
especially if they have been newly converted from primary forests or forest
fallow through logging or slash-and-burn operations. Almost any tree can be
grown on such sites. On the other hand, mast hill farms have long ago been
deprived of protective forest cover and may have been used abusively. In such
cases, the soil is likely to be shallow and depleted, compacted, rocky and
nutrient-poor, and it is usually covered by hardy grasses (e.g.,
Imperata). Under this degraded condition, only the species known to be
hardy enough to withstand poor soils could and should be planted, such as
Acacia auriculiformis and Calliandra calothyrsus (see Table 2).
Other species with less tolerance for poor soils may be planted, but only after
expensive addition of commercial fertilizers.

2. Soil pH - There is a wide variation in soil pH among
hilly lands. Those with limestones, for instance, are alkaline and have high pH
values, while the badly eroded and leached grasslands are often acidic and are
lower than pH 7. After determining soil pH ratings either from local soil
surveys (if available) or by conducting a simple litmus test, the farmer may
choose tree species for his farm. For acidic soils, Acacia auriculiformis
and Acacia mangium may be used. For alkaline soils, on the other hand,
Leucaena or Sesbania may be planted. Calliandra is more or
less neutral with regards to pH (see Table 2).

3. Elevation and temperature-Within the tropical
zone, site elevation heavily influences temperature. For this reason,
altitudinal limits for trees are in fact temperature limits. For example,
species that require or can only thrive in high temperatures (e.g.,
Cassiasiamea) must be planted at low elevations (e.g., not higher
than 500 m), while those that thrive in, or can tolerate cool temperatures can
be planted at higher elevations (e.g., Acacia mearnsii). Because latitude
or distance from the equator likewise affects temperature, altitudinal limits
for species will vary depending on the latitude of the site. For instance, in
the northern Philippines (latitude about 20° N), Leucaena can be
grown only up to about 400 m, while close to the equator, in Papua New Guinea
(latitude about 50 S), the same species has been observed to grow well up to
1,200 m.

4. Soil moisture - This factor is directly influenced by
rainfall intensity and seasonal distribution. Legume tree distribution is, in
turn, influenced by soil moisture. In the wet tropics, species that can stand
rainfall of 1,500 mm or greater per year (e.g., Gliricidia sepium) may be
suitable. In the arid tropics (Africa, South Asia) drought-resistant species
(e.g., Prosopis spp., Albizia lebbek) may be selected. Because
rainfall in most areas is seasonal, species to be planted must have a wide range
of moisture adaptability ranging from drought-resistance to ability to survive
temporary water logging. Many tree legumes have this quality (Table
2).

Compatibility With Locally Preferred Food Crops

Upland farmers have their own nutritional preferences which, to a
certain extent, are influenced by sociocultural factors and by the natural
environment in which their food crops are raised. In wet lowlands, for example,
farmers raise and eat paddy rice. In dry uplands, on the other hand, less
moisture-demanding cereals such as corn or upland rice may be cultivated. In
regions where winds are relatively calm, farmers may grow more crops that have
above-ground products. In areas severely affected by strong winds, however, they
may cultivate root crops. Once affinity for given food crops is established, the
farmers hardly change crops to suit the intercropped trees. Instead, trees are
selected that are compatible with the food crops.

Different food crops have varying requirements for sunlight,
moisture, temperature, and nutrients. Thus, they will react in different ways
when interplanted with various trees, which themselves have dissimilar
characteristics. Some food crops, such as upland rice, are light-demanding. If
this characteristic is known beforehand, the farmer may select tree species that
have thin canopies that will allow as much sunlight as possible to reach the
rice crop, and to space trees as widely as possible so as not to defeat the
yield-increasing purposes of integrated tree-food cropping. On the other hand,
food crops exist that have a tolerance or need for partial shade in order to be
productive. Coffee and cocoa, as well as Dioscorea, are well known for
their partial shading needs and tolerances. In this case, legumes with denser
leaves, such as Acacia and Calliandra, may be chosen for
intercropping, or thin-canopied species (e.g., Albizia) could be spaced
more closely.

Food crops benefit from the nitrogen supplied by leaf drops or
herbage of nitrogen-fixing legume trees with which they are intercropped. It has
been observed that there are certain food crops, however, such as rice and
coffee, which, when supplied excessively with nitrogen fixed by trees, grow
rapidly but fail to bear flowers and seeds. Clearly, the legume tree compatible
with these crops should be one with a lower level of nitrogen yield.

In order that the trees will not compete with, and deprive, the
food craps of nutrients and moisture, deep-rooted species must be chosen for
agroforestry use so that the annual food crops will subsist on the moisture and
nutrients on the top soil layer while the trees will absorb those in the lower
soil horizon.

Use of and Market for The Tree Products

There are many products or end uses for legume trees grown on
agroforestry farms: fuelwood, poles, and pulpwood from the stems and branches;
food and fodder from the foliage and fruits; and fertilizer (green manure) from
the leaves. In general, because many upland farmers cannot afford to wait long
and must harvest their trees early, the wood products they extract are small and
usually would not be suited to the commercial production of sawn timber and
plywood. Thus the tree branches may be used only as fuelwood, poles, or
pulpwood.

The intended principal end use or combination of end uses will
influence the selection of legume-tree species. For example, if the principal
use is domestic fuelwood, the tree species selected must be: (1)
straight-grained for easy splitting; (2) hard so as not to burn too quickly; (3)
easy to ignite; and (4) not smokey when burned. Such legume species as
Calliandra, Leucaena, and Gliricidia may be chosen.

If the end use is poles for fencing or for propping up fruit-heavy
banana plants on commercial plantations, as in the southern Philippines, the
species must have long, straight, strong, durable stems. Leucaena appears
to be a suitable species.

If the wood is intended for sale to a pulp and paper factory, the
wood must be light-colored, long-fibered, light-weight for easy handling, and
easily debarked. Albizia falcataria is an appropriate species to raise
for this purpose.

Finally, if the principal product is fodder, a species with
palatable leaves and a rapid rate of leaf regrowth after cutting, such as
Calliandra or Leucaena, may be selected.

Availability of Planting Stock

A tree species may possess all the characteristics highly desired
by farmers, but if seeds, seedlings or cuttings for planting are dot readily
available, that species may not be adopted. A case example is the Giant
Leucaena from Hawaii. At the early stage of its distribution, its
incredibly high growth rate so impressed Southeast Asian farmers they created
such a large demand that not enough seeds were available and seed prices soared
beyond their reach. In the end, most of the farmers settled for the bushy or
"dwarf" variety of the species, which is not as productive as the
"giant."